In consideration of the fact that there is an ongoing increase in chemical, biochemical and genetic analysis in assays, a strong demand for the automated pipetting of liquid cellular samples can be observed.
Different types of pipetting units and pipetting methods have been implemented in various analytical instruments. Among the various pipetting methods, the most challenging ones are those requiring aspirating a volume of test liquid through a cap of a test liquid container. This is typically done by piercing with the nozzle of the pipetting unit a septum of elastomeric material sealing a test liquid container in which the test liquid is contained. Precision of volume and reproducibility may be particularly hard to achieve when the test liquid container is partially evacuated and/or the volume to be aspirated is small, e.g., below 5-10 microliters. This is due to the fact that air may be present in the pipetting unit and also to the fact that negative pressure, i.e., a pressure lower than atmospheric pressure, is present in the liquid container. This negative pressure, which may be different from container to container, may have an effect on the air in the pipetting unit. This in turn may affect the actual volume of liquid being aspirated and the position of the aspirated volume in the pipetting unit. This means that a first pipetting error may occur when aspirating (a wrong volume) and a second pipetting error may occur when dispensing (wrong position), possibly resulting in only part of the aspirated test liquid being dispensed or no test liquid at all being dispensed.
In order to improve precision of pipetting, the pipetting unit may be operated with a pressure transmission liquid, in which the presence of air in the pipetting unit is minimized. However, a small amount of air may be still dispersed in the pressure transmission liquid, e.g., in the form of microbubbles. Air is typically present also at the extremity of the nozzle, e.g., due to evaporation of pressure transmission liquid or on purpose by aspirating a plug of air in order to separate the pressure transmission liquid from the test liquid to be aspirated. This air, which is affected by the pressure conditions inside the container, may be the major responsible factor for pipetting errors especially for small volumes as mentioned above.
Nowadays automated cellular analyzers such as hematology analyzers, e.g., 3 or 5 part differential hematology analyzers, or other analyzers and pipetting robots pipetting cellular samples need to pipette cellular samples precisely, accurately, safely and contamination free. Such cellular samples are, e.g., blood, diluted blood, processed blood products, other body fluids containing cells such as suspensions of cells received from bone marrow, cerebrospinal fluid (CSF), urine, suspensions of cells received from smears, or other cell preparations or cell culture media containing cellular matter. Such cellular samples are often held, provided and processed in tubes or other suitable containers, such as, e.g., capped or uncapped blood collection tubes, plastic tubes, coated glass tubes, etc.
As, on one hand, the pipetting and pressure transmission liquid, on such automated analyzers, is done under full control of the automat and is generally unattended by a human user, and on the other hand, such analyzers produce in many cases results which are critical, such as a health status of a person, the safe, reliable, accurate, precise and cross-contamination-free operation is a must. An efficient means to support this is the use of capacitive level detection. As a capacitive level detection, a means which is able to detect the upper surface of a sample is to be understood, which helps the analyzers to locate the sample's upper level and consecutively conduct a safe, precise and accurate pipetting while taking into consideration the upper and lower level of the sample. Anyhow, many cellular analyzers do not use such means. Some analyzers use a fixed immersion depth for the pipetting tip. In such systems, often only a completely absent sample is detected. Other analyzers that use capacitive level detections use as pressure transmission liquid deionized water, which is not compatible with cellular samples, leading to lysis of cells under the hypotonic conditions at the interface of the sample and the pressure transmission liquid. Other analyzers using liquid level detections use isotonic solutions of ionic components that are prone to electromagnetic disturbances, which can cause errors in finding the upper level of the sample.
The use of different liquids in the pipetting unit is rather complicated, particularly in view of the liquid level detection by means of catching the variance of capacitance.